1. Field of the Invention
The invention relates to the field of printing technology, in particular to register control in web-fed rotary printing machines.
2. Discussion of Background
The invention is employed to particular advantage in shaftless rotary printing machines. A shaftless rotary printing machine with rubber blanket and plate or forme cylinders combined in pairs to form cylinder groups is described in DE 43 44 896 A1. European Patent Application Number 98101727.0, not previously published, describes a method for regulating the drive for transporting a paper web of a printing machine, said method being suitable particularly for shaftless rotary printing machines. According to this method, the regulation of the rotational speed of the elements determining the web tension is lowered or raised as a function of a load torque in accordance with a load characteristic, so that the drive simultaneously adjusts the command variables, namely the rotational speed and driving torque or web tension, and so that the leading desired value is compensated in accordance with the load characteristic.
An essential advantage of individually driven rotary printing machines is that these machines are capable of carrying out a product change while the machine is running. The method described in the abovementioned European patent application for the most part avoids the web tension changes which normally occur due to printing stations being thrown on and off, as required during a product change, and in the event of a paper type change while the machine is running. The stability thereby achieved in respect of the transport of a paper web of a printing machine is excellent.
Despite the excellent stability as regards the transport of a paper web, there is one problem which it does not solve, or only partially solves, particularly in the case of a product change or paper type change while the machine is running, namely the positioning of the color and cut registers which determines the print quality.
According to the prior art, as shown in FIG. 2, color and cut registers are influenced by independent measuring elements, in that the regulating variables recorded by the measuring elements are linked to the positioning command variables and consequently adjust the individual actuators in terms of the color register and all of the actuators together in terms of the cut register. Where divided webs are concerned, the cut register is adjusted (not illustrated) by means of additional actuators (secondary register). The disadvantage of these devices is that:
during startup operations, the command variables are set at fixed reference values and therefore often require manual actions by the printers. PA1 the measuring elements react only to the stretching of the paper web. PA1 the measurement reaction time due to the dead times during the transport of the web as far as the measuring element, during analysis and during the transmission of data to the control amounts typically to two seconds and the quality of the actual values recorded depends on the smooth running of the web. PA1 the tension/stretching conditions of an entire production having a plurality of webs are considerably influenced by the mutual reaction of the webs running over the common collecting roller. PA1 a generally constant behavior of the paper webs results from the controlled correction of all the registers. PA1 the color registers are preadjusted with quick reaction by virtue of the controlled influence exerted on them, PA1 the cut position is preadjusted with quick reaction by virtue of the controlled influence exerted on all the web tension elements determining the cut position. PA1 less spoilage is produced as a result of improved adherence to color and register in all phases of operation.
The conclusion which may be drawn is that, in the prior art, inaccuracy in register control as regards color and cut is permitted initially, before correcting action is taken on the command variables relatively slowly via additional measuring elements.